Accurately and precisely determining the position and orientation of a drilling assembly during drilling operations is desirable, particularly when drilling deviated wells. Traditionally, a combination of sensors is used to measure downhole trajectory and subterranean conditions. Data collected in this fashion is usually transmitted to the surface via MWD-telemetry known in the art so as to communicate this trajectory information to the surface. Many factors may combine to unpredictably influence the trajectory of a drilled borehole. Accurate determination of the borehole trajectory is necessary to determine the position of the borehole and to guide the borehole to its geological objective as well as avoiding collisions with underground objects, geological features, wells, or zones. In other cases, it is desired to intercept underground objects, geological features, wells, or zones.
In some instances, surveying of a borehole using conventional methods involves the periodic measurement of the Earth's magnetic and gravitational fields to determine the azimuth and inclination of the borehole at the bottom hole assembly. Historically, this determination has been made while the bottom hole assembly is stationary. Consequently, the along-hole depth or borehole distance between discrete survey stations is generally from 30 to 60 to 90 feet or more, corresponding to the length of joints or stands of drillpipe added at the surface. Error accumulated between multiple survey stations caused by, for example, the presence of physical factors or anomalies may skew measurement accuracy. For example, MWD systems which rely on magnetometers may be influenced by magnetic interference both on and off the drill string. Additionally, gyrocompasses naturally tend to lose accuracy at higher inclinations which may reduce overall survey accuracy in gyrocompass MWD systems.